Many applications, including power generation and energy conversion, necessitate high crystallinity and controlled mesoporosity in the production of metal oxide materials. For instance, highly crystalline mesoporous materials with a transition metal oxide framework have immense potential for applications such as catalysis, photocatalysis, sensors, and electrode materials—because of their characteristic catalytic, optical, and electronic properties. With respect to these and other applications, the usefulness of mesoporous materials can only be maximized if they can be produced in a highly crystalline state. For example, when a material composed of mesoporous titanium dioxide is incorporated into a photocatalysis device, any amorphous regions that are present in the material will reduce the device's efficiency by providing trap sites where the recombination of photo-excited electrons and holes may take place (Ohtani, B., Ogawa, Y. & Nishimoto, S. J. Photocatalytic Activity of Amorphous-Anatase Mixture of Titanium (IV) Oxide Particles Suspended in Aqueous Solutions. Phys. Chem. B. 101, 3746 (1997)). Consequently, it is desirable to synthesize highly crystalline mesoporous titanium dioxide for use in photocatalysis applications. High thermal and mechanical stability are other advantages associated with materials produced in a highly crystalline form. Unsurprisingly, then, the synthesis of organized mesoporous materials has stimulated extensive research over the last decade. Despite many efforts, it remains a major challenge to successfully convert the amorphous walls of the organized mesoporous materials (i.e., the “as-made materials”) to crystalline walls during the crystallization process; while simultaneously retaining the original mesostructure. But even after a decade or so of research the direct synthesis of highly crystalline mesoporous metal oxides that are thermally stable and well-ordered still constitutes a major challenge. The conventional processes for preparing crystalline mesoporous metal oxides are: soft templating and hard templating.
Soft templating utilizes soft organic materials, such as block copolymers, to structure direct metal oxides into organic-inorganic hybrids. The unlike blocks in block copolymers tend to phase separate into microdomains forming different mesophases. The size scale of the domains is governed by the chain dimensions and the block ratio determines the mesophase structure. These organic-inorganic hybrids can have hexagonal morphology, but other morphologies such as lamellae, spheres, gyroids, and complex cubic structures such as bicontinuous double diamond are well-known (See, e.g., Yang, P., Zhao, D., Margolese, D. I., Chmelka, B. F. & Stucky, G. D. Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks. Nature 396, 152 (1998)). The organics of the organic-inorganic hybrids are subsequently removed by heat treating (e.g., by calcining) in air to produce a mesoporous oxide structure. A disadvantage of this approach, however, is that the resulting mesoporous oxides cannot be heat treated to temperatures above about 400° C. without inducing the collapse of the mesoporous structure. As a result, rather than enabling mesoporous oxide structures having highly crystalline walls to be synthesized, soft templating is limited to the synthesis of mesoporous oxide structures having amorphous walls that have nanocrystals embedded therein.
Hard templating, on the other hand, utilizes inorganic precursors that are filled into pre-ordered hard mesoporous silica or replicated carbon templates, which are removed in a subsequent step (See, e.g., Jiao, F., et. al. Ordered mesoporous Fe2O3 with crystalline walls. J. Am. Chem. Soc. 128, 5468 (2006). It is often necessary to repeatedly add inorganic precursors in order to ensure that the pores are completely filled. Hard templating allows for heat treatment at elevated temperatures without structural collapse and, therefore, the synthesis of highly crystalline materials. By controlling the heat treatment temperature, either nanocrystals embedded in amorphous walls (˜500° C.) or highly crystalline walls (600° C.) can be selectively fabricated (Jiao, F., et. al. Ordered mesoporous Fe2O3 with crystalline walls. J. Am. Chem. Soc. 128, 5468 (2006)). The disadvantage of hard templating, however, is that it is quite long and tedious since much effort and numerous steps are required to make the template (see, e.g., Lee, J., Kim, J. & Hyeon, T. Recent progress in the synthesis of porous carbon materials. Adv. Mater. 18, 2073 (2006)). A further disadvantage of hard templating is that the removal of the template can be cumbersome and often involves the use of hydrofluoric acid. Furthermore, it is difficult to completely fill the pores of the hard template, even when multiple-impregnation methods are used.
Therefore, a considerable drawback of utilizing either soft or hard templating for the synthesis of highly crystallized mesoporous metal oxides is that each process requires multiple, tedious steps and often only results in poor structure control. With respect to transition metals, it has been acknowledged that, to date, “[h]ighly crystallized mesoporous transition metal oxides with the original mesoporous structure have not been obtained spontaneously,” i.e., in a simple “one-pot”-type procedure. (Shirokura, N., et. al. Synthesis of Crystallized Mesoporous Transition metal Oxides by Silicone Treatment of the Oxide Precursor. Chem. Commun. 20, 2188 (2006)). Hence, there is a need in the art for a simple, yet facile method for synthesizing highly crystalline metal oxide-carbon composites. In particular, there is a need for a simple, yet facile method for synthesizing highly crystalline transition metal oxide-carbon composites. There is also a need in the art for a simple, yet facile method that allows for the highly controlled and successful conversion of highly crystalline metal oxide-carbon composites to thermally stable mesoporous metal oxides, having highly crystalline mesopore walls, without causing the concomitant collapse of the mesostructure. Catalysis, electrocatalysis and sensing applications would particularly benefit from the availability of such mesoporous materials due to the greater surface-area and better accessibility of the metal.
Citation or identification of any reference in Section 2, or in any other section of this application, shall not be considered an admission that such reference is available as prior art to the present invention.